Since a scanning electron microscope (SEM), which scans the surface of a specimen with a narrowly focused electron beam, detects generated secondary electrons with a secondary electron detector, and displays a detected signal as a change in luminance on a TV monitor, allows observation of an object surface at a higher resolution than with an optical microscope, it is widely used for the measurement of the length and/or observation of foreign materials for the semiconductor wafer pattern that has been further microminiaturized in recent years, as well as for academic research. For the inspection of a semiconductor, the recent demand is for a high resolution of a few nanometers at which a material to be inspected can be observed at acceleration voltages of 1 kV or less, without damaging the material. The resolution of SEMs depends on how narrowly the electron beam can be focused on the specimen. The parameters affecting the diameter of an electron beam include, for example, magnitude of an electron source, variations in the energy of an incident electron beam, convergent angle, chromatic aberration of an objective lens, spherical aberration, and diffraction aberration. Conventionally, higher resolutions have been achieved by ingenuities in the electron optical system, particularly, lowering the reduction rate by increasing the reduction rate of an electron source and combining acceleration electric field and deceleration electric field to optimize the shape of the objective lens. However, it is becoming difficult to increase the resolution of SEMs only by optimizing the lens system.
There is an aberration corrector as a device to eliminate the chromatic aberration and spherical aberration. A basic configuration of this device is described in a paper of Zach (J. Zach and M. Haider, Nuclear Instruments and Methods in Physics Research A363 (1995) pp. 316-325), and other configurations are in the papers of Haider (M. Haider, G. Braunshausen, and E. Schwan, Optik 99 (1995) pp. 167-179), Krivanek (O. L. Krivanek, N. Dellby, A. R. Lupini, Ultramicroscopy 78 (1999) pp. 1-11), and others. The aberration corrector of Zach has functions to correct spherical aberration and chromatic aberration, and comprises four stages of electrostatic quadrupole element, magnetic quadrupole element, and electrostatic octupole element disposed along and about the optical axis. By varying the ratio of intensity of excitation to the electrostatic quadrupole element and magnetic quadrupole element, and electrostatic quadrupole element and magnetic quadrupole element, the trajectory of an electron beam passing on the optical axis can be varied in the x-direction or y-direction independently.
The aberration corrector described in the paper of Haider et al. (M. Haider, G. Braunshausen, and E. Schwan, Optik 99 (1995) pp. 167-179) is a corrector to correct the spherical aberration of the objective lens of a transmission electron microscope (TEM), and comprises a combination of two magnetic field 6-polar elements and two sets of doublet lenses.
The aberration corrector described in the paper of Krivanek at el. (O. L. Krivanek, N. Dellby, A. R. Lupini, Ultramicroscopy 78 (1999) pp. 1-11) is a corrector to correct the spherical aberration of a scanning transmission electron microscope (STEM), and comprises a combination of four magnetic quadrupole elements and three magnetic octupole elements. This spherical aberration corrector for STEM can be considered basically Zach's quadrupole-octupole type aberration corrector in which all of quadrupole and octupole elements are magnetic type.
U.S. Pat. No. 6,552,340 discloses a charged particle-based apparatus provided with an automated aberration correcting function.
In addition, JP-A 351561/2001 discloses an invention that employs an aberration corrector in the ion beam irradiation system of an FIB processing apparatus. Since the ion beam has larger mass and higher energy than the electron beam, according to the invention disclosed in JP-A No. 351561/2001, the aberration correction is achieved by decelerating an ion beam generated in the ion source after accelerating it, and providing an aberration corrector in the deceleration space.